The present invention relates generally to artificial cardiac pacing, and more particularly to improved pacing electrodes for stimulating and sensing electrical activity of the heart, and to pacing lead assemblies incorporating such electrodes.
The sinoatrial (S-A) node of the normal mammalian heart acts as the natural pacemaker by which rhythmic electrical excitation is developed and propagated to the atria. In response, the atrial chambers contract, pumping blood into the ventricles. The excitation is propagated through the atrioventricular (A-V) node, which imposes a delay, and then via the conduction system consisting of the bundle of His and Purkinge fibers to the ventricular myocardium, causing contraction and the pumping of blood from the ventricles. Disruption of this natural pacing/propagation system occurs as a result of aging and disease.
Where the human patient has an abnormally slow or abnormally rapid heart rate, or the rate is irregular, it is customary for the cardiologist to prescribe implantation of an artificial cardiac pacemaker selected according to the specific patient's needs. In its simplest form, the cardiac pacemaker consists of a pulse generator with a battery pack, and a lead assembly. The lead assembly includes a pacing electrode to be positioned in stimulating relationship to excitable myocardial tissue, and an insulated electrical coil interconnecting the pulse generator and the pacing electrode to deliver the electrical pulses to the electrode to stimulate the tissue. The electrical circuit is completed via a second electrode (the indifferent or reference electrode), which is connected to a point of reference potential for the cardiac pacemaker, and through the body tissue and fluids. The stimulating electrode may also be used as a sensing electrode by coupling to a detection circuit to sense the electrical activity of the heart. The entire lead/electrode assembly is often referred to simply as the "lead".
In the instant specification, the pacing electrode is sometimes referred to as the stimulating cathodic electrode, the stimulating electrode, or the cathode, and the indifferent electrode is sometimes referred to as the reference electrode, the anodic electrode, or the anode. It will be understood, however, that electrical activity takes place at each electrode during pacing, and that the coupling may be such that each electrode acts, at different times, as cathode or anode.
The lead of choice for use with the cardiac pacemaker is an endocardial catheter, which is readily inserted transvenously to introduce the stimulating electrode into the cardiac chamber to be paced. In contrast, an epicardial lead requires thoracic surgery to affix the electrode to the surface of the heart. Various forms of active or passive fixation may be employed to maintain the stimulating electrode in proper position relative to the excitable heart tissue, such as sutures (epicardial), a corkscrew or flexible barbs, hooks or tines fastened to the lead in proximity to the electrode.
The cardiac pacemaker may employ unipolar or bipolar stimulation, depending on the preference of the physician and the needs of the patient. For unipolar stimulation, the anode is located remote from the heart, and typically comprises the metal case or portion thereof that houses the batteries, pulse generator and other electronic circuitry of the pacemaker. For bipolar stimulation, the two electrodes are in close proximity, typically with the cathode being at the tip and the anode spaced slightly back from the tip as a ring electrode on the lead.
The cardiac pacemaker may operate in any of several different response modes, including asynchronous, or fixed rate; inhibited, in which stimuli are generated in the absence of specified normal cardiac activity; or triggered, in which the stimuli are delivered in response to specified cardiac activity. In each of these modes, output pulses from the pulse generator are delivered via the lead for electrical stimulation of excitable myocardial tissue at or near the site of the cathode, thereby producing the desired rhythmic contractions of the affected chamber. Since stimulation is attributable to current density, small area stimulating electrodes will suffice. The current required to produce a given current density decreases in direct proportion to the active area of the electrode. Small area cathodic electrodes therefore serve to prolong battery life, and increase the interval between required surgical replacements.
In essence, stimulation requires that the electric field be of sufficient field strength and current density to initiate contraction of excitable myocardial tissue at the cathode site. The minimum electrical impulse necessary to achieve this is referred to as the stimulation threshold. The greater the efficiency of the cathode in impressing the electric field on the tissue, the smaller the amplitude and/or duration of (and the energy contained in) the pacing pulse required to achieve the stimulation threshold. Accordingly, highly efficient, low threshold electrodes conserve energy and prolong battery life. Because greater electrode efficiency reduces energy required for stimulation, it may be a factor in reducing injury to tissue at the stimulation site.
Cardiac pacing may be achieved with anodal rather than cathodal stimulation, but the stimulation threshold is higher because the polarizing force of the stimulating electric field on ions at the surface of membranes of the excitable myocardial cells reduces transmembrane potential on the side of each affected cell furthest from the anode, at a point of relatively lower field intensity; in contrast to reduction of the potential at the near side with cathodal stimulation.
Regardless of the type of pacemaker implanted, from the simple fixed rate device to the complex dual chamber pacing/sensing devices and the latest physiologic pacers, it is important to ascertain that the stimulus is having the desired effect. Pulse generation which causes contraction of the selected chamber is termed "capture", and the method of determining that the pacer stimuli are achieving capture is called "capture verification". Capture verification techniques are based on detecting the potential evoked when the heart is captured. If there is no capture, there is no evoked potential, and the amplitude and/or duration of the stimulating pulse must then be adjusted to assure consistent capture. It follows that each time the heart is paced, the cardiac electrical activity may be monitored to detect the presence of the evoked potential and thereby verify capture.
In practice, however, capture verification is fraught with problems, one of the more significant being of a signal-to-noise nature in which the signal sought to be detected is masked by after-potentials attributable to electrode polarization. After the stimulating pulse is delivered, the electrode must "settle down" to allow detection of the evoked potential indicative of capture. This requires a suitable period of delay, which itself precludes the desired detection. Accordingly, some capture verification techniques seek to filter the signal from the masking after-potential, necessitating additional circuitry and space.